Inverter driven control systems for AC induction motors are known in the art. Such systems are generally employed in cases such as electric vehicles where the electric power for driving the motor comes from a DC source such as a battery. It is known that AC motors are relatively lighter, less expensive, more efficient and more rugged than DC motors of the same power rating. The lack of slip rings and brushes in AC induction motors make them better suited to hostile environments and less likely to generate sparks which may be hazardous. The provision of an inverter having a sophisticated control circuit between the battery supply and the AC motor permits easy control of the AC motor.
It is known in the art that the rotor impedance increases with increasing temperature. This temperature dependent rotor impedance results in differing motor current based upon temperature. The rotor temperature may vary depending upon the conditions of use and in particular is likely to increase during initial use of the motor. As a result there is an uneven application of torque for the same drive to the motor based upon the motor conditions. It is known in the art to provide a temperature sensor on the rotor and to change the motor drive to compensate for this based upon the measured temperature. These systems tend to be costly and have reliability problems.
It is therefore a need in the art to provide a less costly and more reliable manner of compensating the motor drive to provide constant torque for differing rotor temperatures.